The present invention relates generally to heat transfer devices and methods. The invention can find significant applications in a wide range of industries.
A heat transfer device utilizing a two-phase heat transfer mode is very effective in terms of the rate of heat transfer and temperature uniformity. Such a heat transfer device typically comprises an evaporator section where vaporization of the heat-carrying fluid hermetically sealed within the device occurs through the heat transfer from an external heat source into the heat transfer device, and a condenser section where the vapor generated in the evaporator is condensed into liquid through the heat transfer out of the heat transfer device to an external heat sink. The heat transfer device requires a driving mechanism for returning the liquid back to the evaporator from the condenser. Since the liquid within the two-phase heat transfer device is substantially saturated, a conventional pump would encounter the so called cavitation problem, which would prevent the pump from creating a pressure head for circulating the liquid within the heat transfer device. Subsequently, the utilization of a conventional pump in a two-phase heat transfer device is rare unless the size of the heat transfer device is sufficiently large and a sufficient sub-cooling of the liquid at the inlet of the pump can be maintained.
Since the invention of the heat pipe by Grover in 1963 (U.S. Pat. No. 3,229,759), the heat pipe has been studied extensively as a two-phase heat transfer device. Although the heat pipe originally invented by Grover employs the capillary action of a wick structure as the driving force for returning the condensate from the condenser (heat rejection section) to the evaporator (heat receiving section), several other driving forces were also employed. These driving forces that have found significant applications include centrifugal forces (rotating heat pipes) and gravitational force (gravity-assisted heat pipes). In addition, the capillary pumped loop or capillary pumped heat pipe which requires a capillary wick structure only in the evaporator section has also been developed. Although the capillary-wick based heat pipe has found substantial applications especially in aerospace undertakings such as satellite isothermalisation, and the gravity-assisted heat pipe has found significant terrestrial applications such as those in heat recovery units, their performance is not without problems. The magnitude of the capillary pumping action is usually small and is limited by the pressure difference across the menisci in the capillaries. As a result, the heat pipe or capillary pumped loop has difficulty in handling applications involving a high heat flux/high power input. Additionally, the reliability of the wick structure is a major concern. The pumping force in a gravity-assisted heat pipe is also relatively weak and is limited to a maximum 1 G acceleration. Furthermore, the gravity-assisted heat pipe is limited to the terrestrial applications where the gravitational head is available.
Cao and Wang (U.S. Pat. No. 5,454,351, Engine Piston, 1996) developed a reciprocating heat pipe which has a heat transfer mechanism different from those of traditional heat pipes. The reciprocating heat pipe is attached to an axially reciprocating mechanism, such as a slider-crank mechanism of an internal combustion engine, cam-follower mechanism, offset slider-crank mechanism, harmonic motion mechanism, or Scotch yoke mechanism. During the operation, the heat pipe experiences the same reciprocating motion as that of the reciprocating mechanism, which creates a reciprocating motion of the liquid within the heat pipe relative to the heat pipe container. This reciprocating motion of the liquid inside the heat pipe effectively returns the liquid condensate from the condenser section to the evaporator section. The collision of the liquid with the heat pipe interior wall and the rapid mixing of the heat-carrying fluid in the heat pipe also significantly enhance the heat transfer within the heat pipe. The reciprocating heat pipe substantially eliminates the aforementioned heat transfer limitations associated with the heat pipe and produces a substantially uniform temperature distribution along the heat pipe length even under a high heat loading condition in the evaporator section. The application of the reciprocating heat pipe, however, is substantially limited to the heat transfer of a reciprocating element. Since most heat transfer applications involve non-reciprocating elements, the applicability of the reciprocating heat pipe concept is rather limited.
It is therefore an object of the present invention to provide a heat transfer device which attains a reciprocating motion of the heat-carrying fluid inside the heat transfer device under both saturated and unsaturated conditions without requiring a reciprocating motion of the entire heat transfer device, so that the application of the heat transfer device will not be limited to reciprocating elements. The present invention also provides a novel fluid pumping mechanism for heat transfer purposes. Said heat transfer device is coined as the reciprocating-mechanism driven heat loop which comprises a hollow loop having an interior flow passage, an amount of heat-carrying fluid filled within the loop, and a reciprocating driver. The heat loop has at least a heat receiving section, a heat rejection section, and a liquid reservoir. Said reciprocating driver is integrated with the liquid reservoir and facilitates a reciprocating flow of the heat-carrying fluid within the heat loop, thereby, liquid is supplied from the heat rejection section to the heat receiving section under both saturated and unsaturated conditions and a high heat transfer rate from the heat receiving section to the heat rejection section is achieved. A substantial temperature uniformity is also attained when the air is evacuated from the loop and the heat-carrying fluid hermetically sealed within the loop is under a substantially saturated condition. Furthermore, many of the heat transfer limitations associated with a heat pipe or capillary pumped loop are essentially eliminated. Since the heat loop is uniquely associated with a reciprocating flow in the loop, the reciprocating-mechanism driven heat loop is also referred to as the reciprocating-flow heat loop.
According to a preferred embodiment of the present invention, the reciprocating driver is a solenoid-operated electromagnetic driver. The electromagnetic driver comprises a pair of solenoids which are disposed outside the casing of the liquid reservoir in an axial direction of the reservoir, and a piston of magnetic metal disposed inside the reservoir movably in an axial direction of the reservoir. When the circuits of the two solenoids are opened and closed alternately opposite to each other, a reciprocating motion of the piston is induced, which in turn produces a reciprocating flow of the heat-carrying fluid within the heat loop. Because of the relatively large reciprocating stroke of the piston and the relatively large volume of the reservoir compared to the remainder of the interior volume of the loop, the liquid is effectively supplied from the heat rejection section to the heat receiving section of the reciprocating heat loop.
According to another preferred embodiment of the invention, the reciprocating driver is a bellows-type driver employing an external reciprocating mechanism. In this case, part of or substantially entire casing of the liquid reservoir is a bellows. A partition is disposed near the mid section of the bellows, said partition is transverse to the longitudinal axis of the bellows and essentially divides the bellows and the liquid reservoir into two segments. The partition is coupled with an external reciprocating mechanism through a connecting rod, said external reciprocating mechanism can be a solenoid-operated electromagnetic reciprocating mechanism or a mechanical reciprocating mechanism. When the external reciprocating mechanism is in operation, the partition would experience a reciprocating motion along with the bellows, thereby a reciprocating flow of the heat-carrying fluid inside the reciprocating heat loop is effectively produced, and liquid is effectively supplied from the heat rejection section to the heat receiving section of the reciprocating heat loop.